1. Technical Field
The present invention relates to photovoltaic devices, and more particularly to selective emitter photovoltaic device structures which include epitaxial grown layers to repel minority carriers from contact areas.
2. Description of the Related Art
Selective emitter structures improve conversion efficiency of solar cells as compared with single emitter structures. The selective emitter structures repel minority carriers from metal contact regions at the emitter by employing locally high doping levels at the contact regions. However the formation of the locally high doping regions requires long drive-in steps to drive-in dopants at high temperatures.
A single-emitter structure employs a single n+ diffusion to form an emitter and is currently an industry mainstream structure. Single-emitter devices can currently achieve a conversion efficiency range of between about 18-19% in production.
A double-emitter structure employs an n++ diffusion to passivate contacts. This is believed to be necessary to improve efficiency, which in some cases reaches over 20%. However, the double-emitter structure suffers from the time and expense of long drive-in steps to form the n++ regions at high temperatures. The long drive-in steps to form n++ regions at the high temperatures may degrade the lifetime of solar grade wafers by activating impurities or forming boron-oxygen complexes.